The smallest constituent
part of an FFT, it represents a cross multiplication,
incorporating multiplication, sum and difference
operations. The name is derived from the shape of the
signal flow diagram.

Companding

A logarithmic scheme for
sampling analog signals that increases the resolution of
signals with a low amplitude. Common standards include A-Law
and u-Law.

Convolution

An identical operation to
Finite Impulse Response filtering.

Correlation

The comparison of two
signals in time, to extract a measure of their similarity.

Data flow architecture

A multi-processing
architecture where individual processing elements perform
multiple instructions on a many pieces of data.

Discrete Fourier
Transform (DFT)

A transform that gives
the frequency domain representation of a time domain
sequence.

Discrete sample

A single sample of a
continuously variable signal that is taken at a fixed
point in time.

Echo canceller

A filter that will remove
reflected signals on a transmission line that are caused
by impedance mismatches.

Equalisation

A filter that will
compensate for the effects of a communications channel.

Fast Fourier Transform (FFT)

An optimised version of
the DFT.

Finite Impulse Response (FIR)
Filter

A filter that includes no
feedback and is unconditionally stable.

Floating-point

A number scheme that
codes a value with a fraction and an exponent and allows
a high signal dynamic range.

Frequency Division
Multiplexing (FDM)

A communications system
that divides a single channel into smaller ones with
discrete frequency bands.

Frequency domain

The representation of the
amplitude of a signal with respect to frequency.

Frequency Shift Keying (FSK)

A digital modulation
scheme that uses a different frequency to represent
different binary levels.

Full duplex

Communications in two
directions simultaneously.

Gain

Amplification or increase
in magnitude.

Half duplex

Communications in two
directions, but only one at a time.

Harvard Architecture

A microprocessor
architecture that uses separate busses for program and
data, this is typically used on DSPs to optimise the data
throughput.

High pass filter

A filter that allows high
frequencies to pass through.

Hybrid

An analog 2 wire to 4
wire (and vice versa) converter.

Infinite Impulse Response
(IIR) filter

A filter that
incorporates data feedback. Also called a recursive
filter.

Linearity

A measure of the
performance of an ADC or DAC to convert signals with
different amplitudes, to the same degree of accuracy.

Linear Predictive Coding
(LPC)

A speech compression
technique that is based on modelling the vocal tract with
a time varying filter.

Low pass filter

A filter that allows low
frequencies to pass through.

Multi-processing

The division of a process
across several processors to improve the performance of
the system.

Multi-tasking

The division of processor
across several tasks, such that each one is able to
receive its required number of processor cycles.

Multiple Instruction
Multiple Data (MIMD)

See data flow
architecture.

Modulation

The modification of the
characteristics of a signal so that it might carry the
information contained in another signal.

Parallel processing

The execution of tasks in
parallel, either on a single processor via multi-tasking
or across several processors by multi-processing.

Pass band

The frequency range of a
filter through which a signal may pass with little or no
attenuation.

Phase Shift Keying (PSK)

A digital modulation
scheme that uses a constant frequency carrier with a
variable phase.

Pipelining

A technique commonly used
in high performance microprocessors that allows an
instruction to begin execution before previous ones have
been completed.

Pole

Artefact leading to
frequency dependent gain in a signal. Generated by a
feedback element in a filter.

Pulse Code Modulation (PCM)

The effect of sampling an
analog signal.

Quadrature Amplitude
Modulation (QAM)

A variation of PSK that
incorporates AM to increase the number of bits per baud.

Recursive filter

See Infinite Impulse
Response filter.

Resolution

The accuracy of and ADC
or DAC circuit.

Sampling

The conversion of a
continuous time analog signal into a discrete time signal.

Sample rate

The inverse of the time
between successive samples of an analog signal.

Single Instruction
Multiple Data (SIMD)

A multi-processing
architecture where individual processing elements perform
the same instruction on many pieces of data, also
referred to as a systolic array.

Spectrum analyser

An instrument that
displays the frequency domain representation of a signal.

Stop band

The frequency range of a
filter through which a signal may NOT pass and where it
experiences large attenuation.

Synchronous
communications

A communications system
where the data is transmitted and received at discrete
times, which are usually synchronized by a clock signal.

Systolic array

See Single Instruction
Multiple Data.

Time domain

The representation of the
amplitude of a signal with respect to time.

Transducer

A piece of equipment that
converts a physical signal into an electrical signal.

Twiddle factor

The coefficients of the
FFT algorithm, typically a ¾ sine table.

Von-Neumann architecture

A traditional
microprocessor architecture that uses the same bus for
program and data.

z-domain

The discrete frequency
domain, in which the jw axis on the continuous time s-plane
is mapped to a unit circle in the z-domain.

Zero

Artefact leading to
frequency dependent attenuation in a signal. Generated by
a feedforward element in a filter.